New expansion rate measurements of the Crab Nebula in radio and optical

TL;DR

This study provides new radio and optical measurements of the Crab Nebula's expansion, revealing different acceleration histories for the synchrotron nebula and optical filaments, and refining their convergence dates.

Contribution

It offers the first Bayesian analysis of the optical filaments' expansion, showing they are slightly older than previously thought and highlighting differential acceleration between nebula components.

Findings

Radio convergence date: CE 1255 ± 27

Optical filaments' convergence date: CE 1091 ± 34

Synchrotron nebula has experienced stronger acceleration

Abstract

We present new radio measurements of the expansion rate of the Crab nebula's synchrotron nebula over a ~30-yr period. We find a convergence date for the radio synchrotron nebula of CE 1255 +- 27. We also re-evaluated the expansion rate of the optical line emitting filaments, and we show that the traditional estimates of their convergence dates are slightly biased. Using an un-biased Bayesian analysis, we find a convergence date for the filaments of CE 1091 +- 34 (~40 yr earlier than previous estimates). Our results show that both the synchrotron nebula and the optical line-emitting filaments have been accelerated since the explosion in CE 1054, but that the synchrotron nebula has been relatively strongly accelerated, while the optical filaments have been only slightly accelerated. The finding that the synchrotron emission expands more rapidly than the filaments supports the picture that the latter are the result of the Rayleigh-Taylor instability at the interface between the pulsar-wind nebula and the surrounding freely-expanding supernova ejecta, and rules out models where the pulsar wind bubble is interacting directly with the pre-supernova wind of the Crab's progenitor.